With the growth of modern technology, improved temperature control systems have also been sought for maintaining a thermal load at a precise temperature under energy intensive conditions. Many such control systems also are required to change the temperature of the thermal load in accordance with process conditions, sometimes with great rapidity. As one illustration, semiconductor manufacturing equipment and processes are often dependent upon temperature control of the wafers or other elements on which various surfaces are being deposited or etched, using techniques which are highly energy intensive. It is thus often necessary to maintain a large semiconductor wafer which serves as the base for formation of thousands of minute complex integrated circuits, under precise temperature control, as the wafer is processed, as under plasma bombardment. By such processes, minute patterns may be selectively deposited or etched in the wafer surface.
Semiconductor manufacture is referenced here merely as one example of one process in which there is a need for precise temperature control under dynamic conditions. Other processes in which there are current or prospective demands for such capabilities will present themselves to those skilled in the art.
In the past, temperature stability in the item being processed has often been achieved by using particular fluids and geometries to define effective heat sinks, for withdrawing or supplying thermal energy from the operating zone as needed, to establish a desired effective temperature level in the item. It has been common, heretofore, to employ a thermal transfer medium which remains typically liquid throughout the entire temperature range used in a process. This medium can maintain adequate thermal transfer capability and at the same time avoid the complexity and unpredictability that would be introduced if a change of phase from liquid to vapor were to be introduced, wholly or partially.
Although the state of the art has been constantly evolving, few distinctly different methods were employed until a novel thermal control technique was introduced by Kenneth W. Cowans et al employing energy transfer using different phases of the same medium. Patents entitled “Thermal Control System and Method” (U.S. Pat. Nos. 7,178,353 and 7,425,835) have issued on this concept and are assigned to the assignee of the present application. This concept employs the thermodynamic properties of a refrigerant in both vapor and liquid phases, properly interrelated to exchange thermal energy with a load so as to maintain the temperature at a selected target level within a wide dynamic range. Consequently, the refrigerant can heat or cool a product and process, such as a semiconductor wafer of large size, at a single or a succession of different target temperatures. This concept has been referred to for convenience by the concise expression “Transfer Direct of Saturated Fluid”, abbreviated TDSF. This descriptor recognizes and in a sense summarizes the operative sequence, in which a medium is first compressed to a high temperature gaseous state, then divided, under control, into two interdependent flows. One flow path maintains the fluid in high pressure gaseous phase, but in this flow path the flow rate and mass are varied in accordance with the target temperature to be maintained. Variation of the one flow affects the differential flow in the other path, in which the refrigerant is converted, by cooling, to liquid phase and the flow is then further cooled by expansion. In this path the flow rate is dependent on the heat load presented to the system. Typically, the flow in this liquefied path is regulated by a standard refrigeration thermo-expansion valve (TXV).
As disclosed in the referenced patents, the two flows, of high pressure gas and cooled expanded fluid/vapor, are recombined in a mixer before delivery to the thermal load. The target temperature for the load is established by adjusting the balance between the two flows by admitting a selected amount of hot gas flow, controlled such that needed pressure, temperature and enthalpy are maintained in a continuous loop.
The TDSF concept has numerous advantages. Some can be best expressed in terms of the range of temperatures that can be encompassed from hot (entirely pressurized gas) to maximum cooling (entirely expanded vapor). The concept also enables the load temperature to be maintained with precision. The target temperature can be adjusted bi-directionally and rapidly.
The use of a refrigerant having a temperature/pressure transition that is somewhere in mid-range relative to the operating temperature band, however, creates possibilities for undesired changes in refrigerant state under certain operating conditions. Situations have been encountered in which performance limitations have been imposed on TDSF systems because of installations which introduce substantial pressure drops or long transport lines for the refrigerant. These conditions can arise because, in a two-phase medium, pressure drops are also accompanied by temperature variations. For example, long line lengths from compressor and condenser units to a semiconductor processing site may be required for operative or geometrical considerations. Heretofore, installations which have inherently required the use of long transport distances for refrigerant media have sometimes imposed restraints on the use of the TDSF concept or the use of special expedients which add undesirable complexity and cost. It is also true that long lines can introduce another complication, that of ‘puddling’: If this occurs, the liquid phase can separate from the two-phase mixture creating variations in mass flow at the line's end. This can adversely alter control characteristics due to surging conditions as pure liquid and pure gaseous phases alternate with mixed two phase flow.